(1) Field of the Invention
The present invention relates to a semiconductor laser adapted to be used as a light source for optical measuring and solid-state laser excitation, and is capable of operating with a large output of up to several tens of Watts, and more particularly to a semiconductor laser and a composite semiconductor laser in which a large output can be obtained with a narrow optical output region.
(2) Description of the Related Art
In the field of, for example, optical measuring, a device for measuring distance to a given point has been developed. For such a device, there is a need for a high output laser diode as a light source which generates an optical output in the Watt and several tens of Watts classes.
For showing structural differences in the semiconductor laser according to the invention as compared with prior art semiconductor lasers, FIG. 1A shows the general structural arrangements of a laser outputting elements according to the invention and FIGS. 1B-1D show those of prior art examples.
In FIGS. 1A-1C, the numeral 9 depicts a light gain region, 10 depicts a high reflective film, and 11 depicts a low reflective film.
A first prior art example (FIG. 1B) relates to a high output laser diode which generates an optical output in the Watt class and in which, in a single diode chip, the width of the light emission region is made broader in order to obtain high power. This is called a "broad area laser diode". According to a report by H. Yamanaka, et al, under the title "Super High Power Semiconductor Laser," in Laser Studies, 1990, Volume 18, p. 555, output of 6 W has been obtained in the emission region with 600 .mu.m width. Also, there is a report on a device in which a plurality of laser elements are arranged in an array form, according to a report by Harnagel, et al. Under the title "High-power quasi-cw monolithic laser diode linear arrays", in Appl. Phys. Lett., 1986, Volume 49, p. 1418, with which output larger than 100 W has been obtained in the total emission region with 7200 .mu.m width.
In this first prior art region example (FIG. 1B), the width of the active region is 100 .mu.m, and the length of the action region is 1000 .mu.m.
A second prior art example (FIG. 1C) relates to a laser diode whose active layer width is flared in the direction of a resonator so as to provide stable high power with a fundamental transverse mode up to the Watt class. Such a prior art example, as shown in a perspective view and plan view, respectively, in FIGS. 2 and 3, has been reported by K. Shigihara, et al. under the title "High-power and fundamental-mode oscillating flared SBA lasers," in Electronics Letters, 1988, Volume 24, No. 18, pp. 1182-1183.
In this second prior art example, the width of the active region at the light output side is 100 .mu.m, the width of the active region at the rear facet side is 4 .mu.m and the length of the active region is 1000 .mu.m.
The semiconductor laser of the second prior art example, having a desired flared configuration, is formed through processes wherein a buffer layer 2 and current blocking layer 3 are grown on substrate 1, a part of the current blocking layer 3 is etched and removed such that the width thereof varies in the direction of a resonator and, on the entire surface of the resulting structure, a lower cladding layer 4, an active layer 5, an upper cladding layer 6 and a contact layer 7 are sequentially formed one over another. Then, electrodes 8a and 8b are formed respectively at the epitaxially grown layer side and the substrate side, and a highly reflective film is formed on a facet at a narrow side of the etch-removed portion of the current blocking layer 3. The current is injected only to the etch-removed portion of the current blocking layer 3 so that, as seen in plan view in FIG. 3, the widths of the light emission region 9 vary in the direction in which the light is resonated (thus, flaring in the direction to the light output). By using such an element, 4 W output has been obtained in the light output region with 200 .mu.m width according to a report by E. S. Kintzer, et al. under the title "High-Power, Strained-Layer Amplifiers and Lasers with Tapered Gain Regions" in IEEE Photonics Technology Letters, 1993, Vol. 5, No. 6, pp. 605. In this prior art example, the optical output at the wide region and that at the narrow region are the same as each other.
A third prior art example (FIG. 1D) relates to an arrangement wherein an amplifier is of an integrated Master Oscillator Power Amplifier (MOPA) which is reported, for example, by R. Parke, et al under the title "2.0 W CW, Diffraction-Limited Operation of a Monolithically Integrated Master Oscillator Power Amplifier" in IEEE Photonics Technology Letters, 1993, Vol. 5, No. 3, pp. 279-300. According to the report, output larger than 2 W has been obtained in the light output region with 200 .mu.m width.
In this third prior art example, the width of the active region at the light output side is 100 .mu.m, the width of the active region at the rear facet is 4 .mu.m, and the length of the active region at the amplifier section is 1000 .mu.m. That is, in this example, the laser beams formed at the DFB laser region 16 are amplified at the amplifier region 17 and emitted through an antireflection film 18.
FIG. 4 shows electric current/optical output characteristics of each of the structures shown in FIGS. 1A-1D. Since the reflectivities of the reflective mirrors in the structures of FIGS. 1A-1C are respectively the same, it is assumed that the respective internal losses are also substantially the same and that efficiencies are substantially the same, accordingly. Further, output of light increases along with the injection of current, but there is a limit in the amount of light output since saturation of light output becomes distinct beyond a certain current density. Experimentally, such a current density was estimated as 66 kA/cm.sup.2 (the point of the current density 66 kA/cm.sup.2 is indicated by the mark V in FIG. 4). Where current density and efficiency are the same, higher output is obtained by the laser with wide light emission region according to the present invention. In actuality, whereas, at the current density 66 kA/cm.sup.2 at which saturation of light output begins, the output obtained in the laser according to the present invention is 50 W, the outputs obtained in the prior art examples 1 and 2 are 32 W and 27 W, respectively.
In the amplifier integrated type laser shown in FIG. 1D, the light output is limited by the saturated output at the amplifier section. Here, the reflectivity at the facet was made 0.001% and the maximum injection current was made 4 A. In the semiconductor laser having a configuration of this type, it is difficult to obtain an output larger than 10 W.
From the above, it can be appreciated that, with the flared type semiconductor laser according to the invention, a larger output can be obtained in the narrow light output region as compared with each of the conventional examples explained above.
In the first prior art example, in which it is intended to increase the output by broadening the width of the light output region or by increasing the number of semiconductor lasers through an array arrangement, there is a problem that, since the light emission region is required to be large, the optical system for forming, for example, a measuring device inevitably becomes large.
In the second prior art example which relates to a flared structure type semiconductor laser, the light is outputted from the portion with a wider active region, and output is made large by increasing the width of the light emission region at the portion with a wide active region. Thus, because of mode control, there is a limitation also in widening angle of the light emission region such that, in order to increase output, it is necessary to make the length of the resonator. Such an attempt to increase the output results in the lowering of electric/optical output conversion efficiency. Therefore, a problem in the second prior art example is that, in application to an optical measuring device, capacity of the current source for driving the laser is required to be large.
In the third prior art example, because the oscillation at the amplifying region is suppressed, it is not possible to increase current density such that, in order to increase light output, it is necessary to increase the length of the resonator or the width of the light output region. In the case of increasing the resonator length, because of increase in light loss at the amplifier region, the capacity of the current source for driving the laser is required to be as large as in the second prior art example. In the case of increasing the width of the light output region, the overall light emission region becomes large such that the optical system, for example, for an optical measuring device, inevitably becomes large.